The use of NMR techniques in measurement, detection and imaging has become desirable in many scientific fields of endeavor. The non-invasive, non-destructive nature of NMR has facilitated application to industrial instrumentation, analysis and control tasks, in a variety of applications, including but not limited to cosmetics, perfumes, industrial chemicals, biological samples and food products. As one example, check weighing is used by the pharmaceuticals industry for monitoring and regulating the amount of drug in a sealed glass vial during filling. The drug weight can be as small as a fraction of a gram, and is required to be weighed with an accuracy of a few percent or better, in a vial weighing tens of grams at a rate of several weighings per second.
International Patent Application No. WO 99/67606, incorporated herein by reference as if fully written out below, describes a check weighing system for samples on a production line using NMR techniques. This system includes a magnet for creating a static magnetic field over an interrogation zone to produce a net magnetisation within a sample located within the interrogation zone, and a RF coil for applying an alternating magnetic field over the interrogation zone to cause excitation of the sample according to the principles of NMR.
As is well known in the NMR art, after pulse excitation of the sample by the alternating magnetic field, the sample emits a signal induced in the RF coil, called the free induction decay (“FID”), from which much information, like sample mass (or weight) can be learned. The temperature of the sample exerts a great influence on molecular activity of the sample, and, in turn its FID. For example, a change in temperature of the sample alters the rate of molecular motion, also altering the effective strength of the dipole—dipole interaction., i.e., proton relaxation, that is central to a nuclear magnetic resonance interaction and emission of the FID. Moreover, if the sample excitation produces incomplete magnetization in the sample at the time of the sample's magnetic resonance measurement, the temperature influence on the magnetization process itself greatly enhances the effect of temperature variations in the sample. Consequently, if the sample mass or weight (hereinafter referred to as “sample weight”) is to be determined with accuracy and precision, the temperature of the sample at the time of its magnetic resonance measurement must be accurately measured or determined.
A direct measurement would seem most advantageous to find the temperature of the sample at the time of its magnetic resonance measurement. However, as noted above, the principals of magnetic resonance measurement require that at the moment of measurement the sample be positioned in both the interrogation zone and the alternating magnetic field region of the RF coil. This geometry, and the essential need for magnetic field homogeneity, makes it very impracticable to place one or more temperature sensor devices so that the temperature of the sample may be accurately measured at the time of magnetic resonance measurement of the sample.
This leaves the alternative of measuring sample temperature at some point on the moving production line either before or after the interrogation zone, and using that information to find the required sample temperature at the time of its magnetic resonance measurement. Previously in the art sample temperature was measured at the sample filling station, and the sample temperature assumed to be constant between the times of temperature measurement time and magnetic resonance measurement. Because sample temperature does vary during this time interval, such NMR sample weight determinations were less accurate.
It is desirable to provide a method for more accurately determining the temperature of samples on a production line in a NMR check weighing system at the time the magnetic resonance measurement of the sample is made.